The plastic substrate for a high-density information recording medium for use in an optical recording-reproducing apparatus (referred to as an optical disk substrate for short) is produced by photopolymerization process or injection molding. Since the former process requires an UV apparatus for curing and is low in productivity, research and development efforts have been directed to the production by injection molding.
The raw materials for injection molding which are preferred because of transparency and stability include acrylic resin, polycarbonate resin, polyester resin, and methylpentene resin, with the first two being in general use. The resins are required to meet the strict requirements regarding optical properties (e.g., transparency and birefringence), physical properties (e.g., impact strength and dimensional stability), chemical properties (e.g., moisture absorption), and moldability (e.g., flowability and transferability).
The optical disk substrate should satisfy all the requirements so that it permits the recording of information in the form of submicron pits each smaller than individual particles of cigarette smoke. The details of these characteristic properties are described in "Nikkei Electronics", 1982, 6-7, pp. 133-152, published by Nikkei McGraw-Hill Co., Ltd.
As mentioned in this literature, the optical disk has already been put to practical use. For example, acrylic resin is used to make large video disks and DRAW disks up to 300 mm in diameter, and polycarbonate resin is used to make small compact disks up to 120 mm in diameter. Although acrylic resin can be made into large disks (300 mm in diameter) because of the low melt viscosity, ease of injection molding, and low birefringence, the disks made of acrylic resin seem inadequate for storage over a long period of time because they absorb a large amount of moisture, resulting in deformation with time. Moreover, they are liable to heat distortion encountered during metal film deposition.
On the other hand, polycarbonate resin has good resistance to heat distortion caused by metal film deposition and is low in moisture absorption. However, it has a disadvantage in that when it is made by injection molding into a large disk having a diameter of 130 mm or more, the birefringence becomes great to such an extent that marks recorded on the disk cannot be read optically. For this reason, polycarbonate resin has not been used for optical disks of a large diameter.
The fact that polycarbonate resin can be made by injection molding into small compact disk substrates having a diameter of 120 mm or less is already known as disclosed in the above-mentioned literature and Japanese Patent Laid-Open Nos. 126119/1983 and 180553/1983. However, nothing has so far been disclosed as to producing a large optical disk substrate having a diameter of 130 mm or more from polycarbonate resin by injection molding. It is to be noted that if the diameter of the disk is increased from 120 mm to 200 mm, or multiplied by the ratio of 1.7, the area of the disk becomes 2.8 times greater. Thus, for this enlarged optical disk, it is extremely difficult to keep the birefringence of the entire recording surface of the optical disk 25 nm or less in its magnitude and uniform in distribution.